Substantial evidence has demonstrated a role for the Notch gene family in multiple human cancers, including neoplasms of the lymphoid system, pancreas, breast and CNS. This transforming activity is an intrinsic property of Notch, which mediates its effects through a transcriptional regulatory complex. However, there remains a significant lack of knowledge with respect to the molecular events leading to the formation of the Notch transcriptional activation complex. Gamma secretase inhibitors (GSIs) are the only drugs targeting Notch that are being tested for efficacy in clinical trials. However, aberrantly expresse Notch intracellular domain (NICD) does not respond to GSI treatment, turning into a high priority the search of new elements in the notch pathway suitable to be pharmacologically targeted. With this purpose, we have searched for new components in the Notch transcriptional activation complex and have identified a novel Notch-associated protein, we termed Notch Activation Complex Kinase (NACK). NACK functions as a co-activator of Notch transcriptional activity and it is expressed in human tumors but not in the adjacent normal tissue. Furthermore, NACK is required for Notch-driven cell transformation. Our preliminary results convincingly identify NACK as a new member of the Notch transcriptional complex and a putative target to inhibit Notch pathway. The underlying hypothesis of this proposal is that NAK is a transcriptional co-activator implicated in tumorigenesis. Our goal is to characterize th molecular mechanisms regulating NACK function, to determine its role in tumorigenesis and to investigate the prospect of using NACK as a tumor biomarker and a putative pharmacological target in anti-cancer therapies. The specific aims of this proposal are: i) Characterize the molecular basis of the interaction between NACK and the Notch transcriptional regulatory complex~ ii) Characterize NACK function in vivo using a conditional NACK-KO mouse~ and iii) Determine the role of NACK in Notch-driven tumorigenesis. The long-range goal for these studies is to obtain a comprehensive understanding of NACK function in order to contribute to the rational design of cancer therapeutics